This invention relates to a method of manufacturing a porous glass preform for an optical fiber, and more particularly to an improvement in the method of manufacturing a porous glass preform by depositing fine glass particles formed by hydrolysis in a flame produced by a burner.
One of the known methods of manufacturing a porous glass preform for an optical fiber is the vapor phase axial deposition (VAD) method. In this method, as shown in FIG. 2, a mixture of a gas for combustion and a gas of a glass material is jetted out through a burner used for forming glass particles, hereafter simply xe2x80x98burnerxe2x80x99 2 (or 2xe2x80x2) to produce a flame 3 (or 3xe2x80x2) in which the glass material is hydrolyzed or oxidized to form fine glass particles, and while the glass particles are deposited on the lower end of a rotating starting member 6, to form a deposited porous body, the starting member 6 is moved relative to the burner 2 (or 2xe2x80x2) with the growth of the porous body, whereby a porous glass preform 1 is obtained. Although the method shown in FIG. 2 employs two burners, a method using only one burner, or more than two is alternatively possible. The porous glass preform 1 is heated in an electric furnace to form a transparent glass preform and it is drawn into an optical fiber.
In the conventional VAD method the burner 2 (or 2xe2x80x2) is provided with, for example, SiCl4 as the glass material, and a fuel gas such as a hydrogen or hydrocarbon gas, and a gas assisting combustion, such as oxygen or air, as the gases for combustion. Fine glass particles (SiO2) are formed by reaction of the following formula (I):
SiCl4+2H2Oxe2x86x92SiO2+4HClxe2x80x83xe2x80x83(I)
Not all of the glass particles that are formed, however, are deposited as the preform 1. Some of the glass particles float in the reaction vessel 4 and attach to its inner wall to form a layer of glass particles thereon. If this layer grows to some extent in thickness, glass particles are likely to fall off the reaction vessel wall, attach to the surface of the porous glass preform 1 and form a gap therein. In this case, the voids may be formed when the preform is heated into transparent glass.
As a method for solving this problem, the following method has been proposed in Unexamined Published Japanese Patent Applications Nos. 162642/1987 and 123831/1988. A heater and an outlet to a reaction vessel for gas are added, and the gas heated to high temperature is forced to flow around the burner and the porous glass preform along the inner wall of the reaction vessel; thereby the gas is prevented from remaining near the inner wall of the reaction vessel; glass particles are prevented from attaching to the reaction vessel; the gas is caused to flow in the reaction vessel so as to retain the glass particles from floating in the reaction vessel. The burner for synthesizing glass particles also has a problem with respect to the attachment and mixture of glass particles. Namely, when the mixture of the combustion gas and the glass material is jetted from the tip of the burner for synthesizing glass particles, a part of the gas mixture is likely to scatter around the burner and attach to the vicinity of its outlet as glass particles. The glass particles are also likely to even enter the burner as a result of their entrainment by the gas surrounding it. Moreover, even if the floating of glass particles may be restrained during the synthesizing of glass particles, by the method proposed in the above mentioned patent applications, it is still likely that after the manufacturing of a preform is stopped, the glass particles may enter the burner during the cooling of the preform.
If the glass particles which have attached to the burner, or entered it as described above are left as they are, they are likely to leave the burner and attach to the surface of a preform during the subsequent preform manufacturing. In this case also, the particles attach in a manner different from new particles produced in a flame and deposited on the preform, and are likely to form voids when it is heated into transparent glass. Moreover, the attached glass particles spoil the burner if they form transparent glass in the burner under the heat of the gas for combustion. Therefore, it is necessary to clean the burner after manufacturing of each preform by removing, by suction or other means, the glass particles which have attached or entered therein.
Although the methods proposed in the above mentioned patent applications have been somewhat effective for preventing the glass particles from attaching to the inner wall of the reaction vessel of glass particles, the formation of voids in a preform by the glass particles attaching to it is still an outstanding problem.
The glass particles are likely to remain not only in the reaction vessel, but also in the burner even after it is cleaned as mentioned above, and attach to a preform after starting of the synthesizing and become the cause of the voids. Namely, the attached glass particles may fall of the reaction vessel wall or the outlet end of the burner when manufacturing of a preform is stopped, or may not be sucked from the burner completely even by very careful cleaning, and may enter it. Some of the falling glass particles sometimes enter deep into the burner, for example, near its joint to a pipeline, and hence their removal by suction is very difficult. The creating of a sufficiently large pressure difference for removing any foreign matter from such a deep region in the burner by suction is very likely to result in the destruction of its glass wall having a thickness of, say, only 1 mm near its outlet end. The use of a new burner for manufacturing each preform is a very costly solution to such a problem.
Under these circumstances, it is a subject of this invention to provide a method which can prevent fine glass particles from attaching to a burner, or entering it, and thereby avoid the formation of voids in a transparent glass preform.
This subject is essentially attained by starting the deposition of fine particles of glass after causing an inert gas to flow at a rate of at least 25 m/s through a burner for producing those particles.
The inert gas preferably has a pressure elevated above the atmospheric pressure. The pressure of the inert gas is preferably elevated by a pressurizer connected to the burner. The inert gas is preferably caused to flow at a rate of 25 to 50 m/s. When the inert gas is made to flow in the burner for manufacturing glass particles the pressure is preferably reduced by at least about 0.1 kPa in an exhaust pipe extending form reaction vessel in which the burner is mounted.